1. Field of the Invention
In general, the present invention relates to a subscriber-line-terminating apparatus. More particularly, the present invention relates to dynamic allocation of a band required for transmission of information to optical network units.
2. Description of the Related Art
ATM-PON is an access communication system making use of characteristics of both an ATM (Asynchronous Transfer Mode) technology and a PON (Passive Optical Network) technology. The ATM technology makes integrated processing possible by dividing a variety of multimedia services of various kinds such as sounds, pictures and data into cells each having a fixed length. On the other hand, the PON technology makes a system cost low by letting a plurality of users share an optical transmission facility. An ATM-PON system comprises, among others, a subscriber-line-terminating apparatus (abbreviated hereafter to an SLT) installed typically at a telephone office, a plurality of optical network units (abbreviated hereafter to ONUS) installed at buildings and homes and an ATM exchange. The SLT is connected to the ONUs by a splitter called a star coupler, which is a passive device, through optical fibers. A direction from the SLT to an ONU is referred to as a descending direction. On the other hand, a direction from an ONU to the SLT is known as an ascending direction. Information is included in an ATM cell. Information is transmitted in cell units with each cell having additional bytes of data required for communication through a PON on a transmission line. Information is transmitted from the SLT to a plurality of ONUs in the descending direction as a signal comprising successive ATM cells. An ONU receives the transmitted information only after verifying that the information includes data indicating that the information is destined for the ONU. Each ONU sets ATM cells to a time slot obtained information transmission permission by the SLT for avoiding collision so as to transfer the same so that an ascending signal from an ONU to an SLT is commiunicated. The SLT allocates a time slot for an ascending signal to a specific ONU by transmission of a polling signal appending a transmission right for the specific ONU to a descending signal.
Polling in the ATM-PON system is implemented by providing a polling cell called a PLOAM cell at a specific location in a descending frame comprising typically 56 cells. An identifier for identifying an ONU serving as a recipient of a transmission right is set in the PLOAM cell. In this way, a descending signal can be transmitted to a plurality of ONUs. In the conventional system, polling is carried out for allocation of fixed-band portions. In this description, a band portion is a partial band allocated to an ONU. That is to say, a band portion allocated to each ONU is fixed. However, this conventional technique is not capable of well keeping up with a case in which traffic occurs irregularly as a burst as is the case with data traffic. By setting the polling at a time close to the occurrence of a peak rate of information generations, all information can be transmitted even if a burst is generated. With this method, however, polling operations at few bursts will be all wasted so that the transmission band cannot be used with a high degree of efficiency. In order to solve this problem, the polling is set at a time close to an average value of information generations to absorb fluctuations in information generation interval and to avoid congestion. In this case, however, a buffer memory with a very large size is required. In order to solve this buffer-size problem, there was invented a dynamic allocation technique capable of allocating band portions with a high degree of efficiency in a system with a transmission quantity varying dynamically as disclosed in Japanese Patent Laid-open No. Hei 10-242981. This technique is referred to as a conventional method.
In the conventional method, the SLT requests each ONU that the ONU notify the SLT of the amount of information stored in a buffer owned by the ONU by using an ascending signal. The SLT makes such a request by using a descending PLOAM cell. The amount of information stored in a buffer owned by each ONU is referred to as an in-buffer-information amount. In accordance with the descending PLOAM cell, the ONU retrieves the in-buffer-information amount and generates a request for a band portion for transmission of the information. The requested band portions is to be allocated to the ONU by the SLT on a polling basis. The request for the band portion to be allocated on a polling basis is multiplexed with ascending information in an ascending signal, which is then transmitted to the SLT. The SLT extracts the in-buffer-information amount from the request for the band portion to be allocated on a polling basis received from each ONU through a transmission line. Then, the SLT allocates a fixed-band portion determinately allocated on a polling basis to each ONU initially. The SLT then assigns a portion of a shared band allocated on a polling basis as described later to a free time slot. Subsequently, polling information is mixed with descending information in a descending signal transmitted to each ONU. A procedure to allocate a portion of a shared band on a polling basis is explained briefly as follows. First of all, a band unit is allocated to ONUs on a polling basis, that is, allocated to ONUs each having an in-buffer-information amount of at least equal to a threshold value of 0. The threshold value is incremented by 1 and a band unit is again allocated to ONUs on a polling basis, that is, allocated to ONUs each having an in-buffer-information amount of at least equal to a threshold value of 1. Thereafter, each time the threshold value is incremented, the same procedure is executed for the updated threshold value. As the threshold value reaches such a large value that there are no longer ONUs to which a band unit is to be allocated on a polling basis, the threshold value is restored to 0 to start the procedure from the beginning. As a result of the processing, a polling count of an ONU, that is, the number of polling operations in which a shared band unit is allocated to the ONU or the number of shared band units allocated to the ONU, is proportional to the in-buffer-information amount plus 1. Assume for example that the in-buffer-information amounts of ONU 1 and ONU2 are 50 and 300 respectively whereas the total size of the shared band is 1,000 units. In this case, the number of shared-band units allocated to ONU 1 is 143 (=1,000xc3x9751/352) and the number of shared-band units allocated to ONU 2 is 857 (=1,000xc3x97301/352).
However, the conventional method has the following 3 problems.
1: The Size of a Band Portion or the Number of Band Units Allocated to Each ONU Greatly Vibrates with the Lapse of Time.
As described above, the number of band units allocated by the SLT to each ONU is proportional to the most recent in-buffer-information amount plus 1. Thus, almost no band is allocated to an ONU with a small in-buffer-information amount. However, an ONU with a small in-buffer-information amount is not in a state of communicating no information. The small in-buffer-information amount is caused by the fact that a band portion previously allocated thereto by the SLT is greater than the magnitude of the actual communication. Since an ONU with a small in-buffer-information amount is not polled by the SLT, the in-buffer-information amount increases abruptly. When the in-buffer-information amount increases abruptly, the SLT allocates a sufficiently large band portion to the ONU. Thus, the in-buffer-information amount decreases abruptly. These operations to allocate band portions to ONUs are carried out at intervals of updating an allocated band portion. As a result, the size of a band portion allocated to an ONU fluctuates greatly. FIG. 55 is a diagram showing tabular results of allocation based on polling in the conventional system obtained by simulation. In the simulation, the total band is used as a shared band with a size of 3,792 units shared by 5 units ONU 0 to ONU 4. As is obvious from the figure, the size of a band portion allocated to each ONU, that is, the allocation polling count of each ONU on the right side of the table, vibrates greatly with the lapse of time in the downward direction of the table.
The vibration of the size of an allocated band portion causes a fluctuation and a delay in arrival time of information. In the case of such traffic, information leaks in a small-size buffer existing on a transmission line, causing the quality to deteriorate. In addition, the qualities of connection sharing the buffer and the transmission line also deteriorate as well. For this reason, the SLT must be provided with means such as a buffer with a large size for absorbing vibrations prior to transmission to a network, and such a buffer raises the cost of the SLT considerably. In addition, a fluctuation and a delay resulting from a vibration substantially deteriorates the quality of a real-time service such as a service of providing sounds and motion pictures. The problem encountered as a big vibration of a size of a band portion allocated to an ONU is attributed to the fact that the size of a band portion to be allocated to the ONU is simply set at a value proportional to the in-buffer-information amount without computing a correct size of a band portion to be allocated from the in-buffer-information amount.
2: Fair Band Allocation is not Sustained Among ONUs.
Data traffic conforming to a TCP/IP adopted in a network such as the Internet can be TCP traffic subjected to flow control based on the TCP protocol of the upper-level apparatus between user terminals such as personal computers and workstations, or UDP traffic not subjected at all to flow control based on a UDP protocol of an upper-level apparatus. In the TCP traffic, data is transmitted under flow control between user terminals. As a result, the in-buffer-information amount of an ONU is in a stable state of a small value and does not increase to a large value. In the case of the UDP traffic, on the other hand, no flow control is executed. Thus, when communication is started, the in-buffer-information amount increases abruptly. For this reason, if an ONU for only TCP traffic coexists with an ONU for only UDP traffic in the conventional technique, a band portion of a large size is allocated to the ONU for only UDP traffic so that almost no band portion is allocated to the ONU for only TCP traffic. As a result, band portions are not allocated fairly.
3: A Minimum Band Portion is not Taken into Consideration.
In the ATM technology, from a QoS (Quality of Service) point of view, services are classified into classes such as an ABR service and a GFR service. A GFR service is a service for which a minimum band part is set in a shared band portion. When a GFR service is rendered, it is necessary to allocate a shared band portion to each ONU by taking a minimum band size into consideration. With the conventional technique, however, this scheme is impossible.
It is thus an object of the present invention to provide an SLT capable of suppressing a phenomenon in which the size of a shared band portion vibrates.
It is another object of the present invention to provide an SLT capable of sustaining fair allocation of band portions among ONUs.
It is a further object of the present invention to provide an SLT capable of allocating band portions by taking a minimum band size into consideration.
In accordance with an aspect of the present invention, there is provided a subscriber-line-terminating apparatus (SLT) for terminating lines of a plurality of terminal apparatuses and dynamically allocating band portions to the terminal apparatuses by dynamic allocation of time slots to be used by the terminal apparatuses for information-transmission use to the terminal apparatuses and transmission of polling information indicating allocation of time slots to the terminal apparatuses to the terminal apparatuses, the SLT comprising: an in-buffer-information-amount-receiving unit for receiving an in-buffer-information amount indicating an amount of information, which is stored in a predetermined buffer employed in each of the terminal apparatuses and remains to be transmitted, from each of the terminal apparatuses; an insufficient-band-computing unit for computing an insufficient band portion required by each of the terminal apparatuses for transmitting information stored in the buffer on the basis of the in-buffer-information amount received from each of the terminal apparatuses; an apparatus-specific-parameter management table for storing the in-buffer-information amount and a first past-use band variable indicating a band portion dynamically allocated in the past to one of the terminal apparatuses for each of the terminal apparatuses; an in-buffer-information-amount-proportional-band-allocating unit for allocating a band portion corresponding to the in-buffer-information amount to each of the terminal apparatuses on the basis of the insufficient band portion for the terminal apparatus and a total-shared-band upper limit indicating an upper limit of a sum of band portions dynamically allocated to all the terminal apparatuses; a past-use-band-allocating unit, which is used for apportioning a past-use band part into past-use band portions each to be allocated to one of the terminal apparatuses on the basis of the first past-use band variable of the terminal apparatus in case a sum of the band portions proportional to the in-buffer-information amounts of all the terminal apparatuses is smaller than the total-shared-band upper limit where the past-use band part is a difference between the sum of the band portions proportional to the in-buffer-information amounts of all the terminal apparatuses and the total-shared-band upper limit; a first past-use-band-variable-updating unit for updating the first past-use band variable for each of the terminal apparatuses on the basis of a band portion, which is dynamically allocated to the terminal apparatus and includes the band portion proportional to the in-buffer-information amount as well as the past-use band portion, and storing the updated first past-use band variable for each of the terminal apparatus into the apparatus-specific-parameter management table; a shared-band-polling-information-generating unit for generating polling information showing allocation of time slots to the terminal apparatuses on the basis of band portions, which are dynamically allocated to the terminal apparatuses and each include the band portion proportional to the in-buffer-information amount as well as the past-use band portion; and a polling-information-transmitting unit for transmitting the polling information to the terminal apparatuses.
It is desirable to provide an SLT according to the aspect of the present invention wherein the in-buffer-information-amount-proportional-band-allocating unit increases a band portion corresponding to the in-buffer-information amount to be allocated to each of the terminal apparatuses by multiplying each of the insufficient band portions by a coefficient greater than 1 in case the sum of the insufficient band portions for all the terminal apparatuses is smaller than the total-shared-band upper limit.
In another desirable SLT according to the aspect of the present invention, the apparatus-specific-parameter management table includes a guaranteed minimum band portion for each of the terminal apparatuses; there is further provided a minimum-band-guaranteeing unit which is used for computing a guaranteed minimum band part of the terminal apparatuses as a product of the total-shared-band upper limit and a first coefficient smaller than 1 and apportioning the guaranteed minimum band part into individual guaranteed minimum band portions each to be allocated to one of the respective terminal apparatuses on the basis of the guaranteed minimum band portion cataloged in the apparatus-specific-parameter management table for the terminal apparatus in case a sum of the insufficient band portions for all the terminal apparatuses is greater than the total-shared-band upper limit; in case a sum of the insufficient band portions for all the terminal apparatuses is greater than the total-shared-band upper limit, the in-buffer-information-amount-proportional-band-allocating unit multiplies each of the insufficient band portions for all the terminal apparatuses by a second coefficient smaller than the value of an expression [(the total-shared-band upper limit/the sum of the insufficient band portions for all the terminal apparatuses)xc3x97(1xe2x88x92the first coefficient)], and allocates products of the insufficient band portions for all the terminal apparatuses and the second coefficient as band portions corresponding to the in-buffer-information amounts of the terminal apparatuses to the respective terminal apparatuses; and in case a sum of the guaranteed minimum band portions allocated to the terminal apparatuses and the band portions corresponding to the in-buffer-information amounts allocated to the terminal apparatuses is smaller than the total-shared-band upper limit, the past-use-band-allocating unit apportions a past-use band part into past-use band portions each to be allocated to one of the respective terminal apparatuses on the basis of the first past-use band variable of the terminal apparatus where the past-use band part is a difference between the sum of the guaranteed minimum band portions allocated to the terminal apparatuses and the band portions corresponding to the in-buffer-information amounts allocated to the terminal apparatuses and the total-shared-band upper limit.
The above and other objects, features and advantages of the present invention as well as the manner of realizing them will become more apparent, whereas the invention itself will be best understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.